Tetrafluoroethylene fine powder and preparation thereof

ABSTRACT

Tetrafluoroethylene fine powder resins are described which have surprisingly high extrusion pressures and molecular weights which make them useful in post-paste extruded stretching operations. The resins are made by using a permanganate polymerization initiator and controlling its rate of addition.

This is a division of application Ser. No. 621,798, filed June 18,1984,now U.S. Pat. No. 4,576,369.

FIELD OF THE INVENTION

This invention relates to novel tetrafluoroethylene (TFE) fine powderresins, and particularly to such resins that have good stretchperformance.

BACKGROUND OF THE INVENTION

Tetrafluoroethylene (TFE) fine powder resins are non-melt-fabricable andare commonly processed by paste extrusion wherein the powder is mixedwith a lubricant and is then discharged through a paste extruder toobtain films, tubes, tapes, protective coating on wire and the like.

Such paste extruded films, tubes and tapes can be rapidly stretched inunsintered form to form a strong material that is porous to water vaporbut not to liquid water. Such a material is useful in providing"breathable" fabric material for garments, tenting, separatory membranesand the like. Heretofore, the resins useful in making such pasteextruded stretched films exhibited a sensitivity to lubricant loadinglevels and to stretch rates that necessitated careful control over tneloading level used in order to ensure a good stretched product.

It is desirable to improve on these known resins by providing improvedTFE fine powder resins that are not as sensitive to lubricant loadinglevels and which have improved stretchability. This invention isdirected to such resins and to such processes for obtaining them.

SUMMARY OF THE INVENTION

This invention provides an unsintered non-melt-fabricabletetrafluoroethylene fine powder resin characterized in that:

(a) The primary particle size is between 0.1 and 0.5 microns; preferablybetween 0.15 and 0.3 microns,

(b) the specific surface area is greater than 5 m² /g, and preferablygreater than 10 m² /g,

(c) the standard specific gravity is less than 2.190, and preferablyless than 2.160,

(d) the rheometric pressure (sometimes referred to as extrusionpressure) is at least 250 kg/cm², and preferably at least 350 kg/cm²,

(e) the uniformity of stretch is at least 75% throughout a lubricantloading range of 4 weight percent which 4 weight percent range is withina lubricant loading level range between 10 and 25 weight percent at astretch rate of 100%/second.

(f) The uniformity of stretch is at least 75% throughout a stretch rateof between 10 and 100%/second at a lubricant loading level of 17%.

(g) the stress relaxation time is at least 400 seconds measured at 393°C.

In a preferred embodiment, the uniformity of stretch is at least 75%throughout a lubricant loading range of between 17 and 23 wt. % at astretch rate of 100%/second; and also is at least 75% throughout astretch rate of between 22% and 100%/second at a lubricant loading of 17wt. %.

These resins have an unusual insensitivity to lubricant loading levels,have high stress relaxation times (dwell times), and can be stretched atlow stretch rates even at high lubricant levels, whereas prior artresins do not possess this feature.

This invention also provides a process for preparing tetrafluoroethyleneresins by polymerizing tetrafluoroethylene, and, optionally, a smallamount of a selected copolymerizable fluorinated medium in the presenceof a substantially non-telogenic anionic surfactant present in an amountwhich maintains colloidal particles of polymerization product indispersed form, said process being carried out by contactingtetrafluoroethylene and, optionally, said selected comonomer, in thepresence of at least one polymerization initiator defined by the formulaXMnO₄, wherein X is a cation that forms a water soluble salt with theMnO₄ anion (preferably X is hydrogen, ammonium, alkali metal or alkalineearth metal), and wherein the XMnO₄ is added optionally as a prechargeand intermittently or continuously, and where the XMnO₄ last additionoccurs so that the reaction slows down and the end point is at least 5%,preferably 10%, more preferably 20%, longer in comparison with areaction where initiator addition is continued to the end of thepolymerization.

This process produces an aqueous dispersion of the resins of thisinvention. These dispersions are themselves useful for coating metalsand fabrics. On coagulation, the resins are obtained.

DESCRIPTION OF THE INVENTION

The polytetrafluoroethylene resins of this invention are referred to bythose skilled in the art as tetrafluoroethylene fine powder resins. Theterm "fine powder" has attained a special meaning in the art. It meansthat the resin has been prepared by the "aqueous dispersionpolymerization" process. In this process sufficient dispersing agent isemployed and agitation is mild in order to produce small colloidal sizeparticles dispersed in the aqueous reaction medium. Precipitation (i.e.,coagulation) of the resin particles is avoided during thepolymerization.

There is another polytetrafluoroethylene material called "granularpolytetrafluoroethylene resin" which is prepared by polymerizingtetrafluoroethylene by a process in which little or no dispersing agentis employed and agitation is carried out vigorously in order to producea precipitated resin. This process is called "suspensionpolymerization".

The two polymerization procedures produce distinctly different products.The "granular" product can be molded in various forms, whereas theproduct produced by the aqueous dispersion method cannot be molded butmust be fabricated by dispersion coating or by coagulating to obtainfine powder and then adding a lubricant to the powder for pasteextrusion. In contrast, granular resin is incapable of being pasteextruded.

Tetrafluoroethylene may be polymerized alone in the process of thisinvention to obtain a fine powder homopolymer resin of the invention. Inaddition, tetrafluoroethylene may be copolymerized with copolymerizablefluorinated ethylenically unsaturated comonomer provided the amount ofcomonomer is not sufficient to cause the resulting polymer to becomemelt-fabricable or to change the characteristics of the resins of thisinvention.

Representative copolymerizable fluorinated ethylenically unsaturatedcomonomers are represented by the formulas ##STR1## wherein R₁ is--R_(f), --R_(f) --X, --O--R_(f) or --O--R_(f) --X in which --R_(f) is aperfluoroalkyl radical of 1-10 carbon atoms, --R_(f) -- is a linearperfluoroalkylene diradical of 1-10 carbon atoms in which the attachingvalences are at each end of the linear chain, and X is H or Cl; R₂ is F,--R_(f) or --R_(f) --X; and R₃ is H or F. A dioxole may also beemployed, of the formula ##STR2## where Y is ##STR3## and X and X' are For Cl and Z and Z' are each alkyl or fluorinated alkyl of 1-6 carbons.

Representative copolymerizable fluorinated ethylenically unsaturatedcomonomer includes hexafluoropropylene, perfluorohexene-1,perfluorononene-1, perfluoro(methyl vinyl ether), perfluoro(n-propylvinyl ether), perfluoro(n-heptyl vinyl ether), perfluoromethyl ethylene,perfluorobutyl ethylene, ω-hydroperfluoropentene-1,3-hydroperfluoro(propyl vinyl ether), and the like, or mixtures thereofsuch as a mixture of hexafluoropropylene and perfluoro(propyl vinylether). Preferably the comonomers are selected from perfluoro(alkylvinyl ethers) of the formula R_(f) --O--CF═CF₂ ; or perfluoro(terminallyunsaturated olefins) of the formula R_(f) --CF═CF₂ ; or perfluoroalkylethylenes of the formula R_(f) --CH═CH₂, wherein R_(f) is perfluoroalkylof 1-10 carbon atoms.

By the term "non-melt-fabricable" is meant a tetrafluoroethylene polymerwhose melt viscosity is so high that the polymer cannot be easilyprocessed by melt fabrication techniques. Generally the higher themolecular weight of the polymer, the higher the melt viscosity. A meltviscosity above which tetrafluoroethylene polymers arenon-melt-fabricable is 1×10⁹ poises. The melt viscosities ofnon-melt-fabricable polymers are so high that molecular weights areusually measured indirectly by a procedure which gives the standardspecific gravity (SSG) of the resin. The SSG of the resin variesinversely with molecular weight; as the molecular weight increases, thenumerical value of the SSG decreases.

In the process of this invention, tetrafluoroethylene monomer,optionally along with ethylenically unsaturated comonomer, is admixed orcontacted with an aqueous medium containing dispersing agent andpolymerization initiator. The polymerization temperature and pressureare not critical provided the reaction profile recited above is used.Temperatures useful to decompose XMnO₄ are desirable to obtain highmolecular weight near the surface of the resin particles formed. Ideallythe temperature will be between 50°-125° C.; and preferably between65°-100° C. A practical, but noncritical, pressure can be between 15-40kg/cm², and preferably 25-40 kg/cm². The polymerization is ordinarilycarried out in a gently stirred autoclave.

The dispersing agents used are anionic, substantially nontelogenicdispersing agents. Commonly employed dispersing agents are fluorinatedcarboxylates containing 7-20 carbon atoms, such as ammoniumpolyfluorocarboxylates. The amount of dispersing agent present will besufficient to stabilize the colloidal dispersion. It may be ordinarilybetween about 1000 ppm and about 5000 ppm based on weight of wateremployed in the aqueous dispersion. The dispersing agent may be addedprior to initiation of polymerization or may be added in increments asdescribed in Punderson U.S. Pat. No. 3,391,099.

If desired, a paraffin wax (i.e., a saturated hydrocarbon having morethan 12 carbon atoms) that is liquid at the polymerization temperaturemay be employed as described in Bankoff U.S. Pat. No. 2,612,484.Usually, the wax is employed in an amount between 0.1%-12% by weight ofwater in the aqueous dispersion.

Polymerization is effected by mixing the foregoing described ingredientsunder the conditions specified above. Mixing is ordinarily carried outby mildly agitating the aqueous polymerization mixture. Agitation iscontrolled to aid in preventing premature coagulation of resin particlesproduced in the polymerization. Polymerization is ordinarily conducteduntil the solids level (i.e., polymer content) of the aqueous mixture isbetween about 15 and 60 percent by weight of the mixture.

By the term "substantially non-telogenic" used in the definition of thedispersing agent is meant that the polymer produced has an SSG (standardspecific gravity) substantially the same as the SSG of a polymerproduced without the dispersing agent present. SSG is a means ofmeasuring the molecular weight of the polymer produced.

The initiator has the formula XMnO₄ where X is a cation that forms awater-soluble salt with the MnO₄ anion, preferably hydrogen, ammonium,alkali metal, or alkaline earth metal. Preferably the initiator ispotassium permanganate. The initiator may be added to the polymerizationvessel optionally as a precharge, and/or in increments, or continuouslyduring the polymerization, provided that a reducing agent, such asoxalic acid, is preferably present to form a redox couple with theHMnO₄. Oxalic acid can be added as such, but it is also formed in situas a product of TFE oxidation.

The initiator amount added to the polykettle may vary depending on themolecular weight of the product desired. Generally, this amount will be0.1-100 ppm and preferably 1-25 ppm, based on aqueous charge.

The reaction is generally carried out in acidic medium. Succinic acid isa common acid and is preferred because it also prevents coagulation.When the medium is acidic, the XMnO₄ generally forms the acid HMnO₄ insitu. Buffers may be used to control the pH. A complexing agent formanganese, such as a phosphate, may be added to prevent MnO₂ fromforming.

On completion of polymerization, the dispersed polymer particles can becoagulated by high speed agitation. The particles can then be collectedand dried.

Non-melt fabricable tetrafluoroethylene fine powder resins produced bythe process of this invention exhibit excellent stretch performance atelevated temperatures, e.g. 300° C., even at stretch rates below 100%per second, to result in a stretched material that is strong andbreathable but impervious to liquid water. The resins are of highmolecular weight, having an SSG of less than 2.190. They have a highrheometer pressure which is at least 250 kg/cm². They have a primaryparticle size between 0.1 and 0.5 micron. By "primary" is meant the sizeof the colloidal resin particles measured prior to coagulation. Theresins also have a specific surface area greater than 5 m² /g.

In addition, the resins of this invention have several unusual stretchfeatures. First, the resins can be paste extruded over a wide range ofamount of lubricant additive present. Normally fine powder resins aresensitive to the amount of lubricant present during paste extrusion andas the amount is varied, the properties of the paste extruded productwill vary widely. Uniquely, with the resins of this invention, theamount of lubricant can vary widely, e.g. from at least over a loadingrange of 4% within a total range of 10 wt % to 25 wt % with nosignificant loss of stretch uniformity and smoothness of surface at astretch rate of 100%/second. This is an insensitivity to organiclubricant loading levels that is not ordinarily seen in other finepowder resins. Suitable organic lubricants include hexane, heptane,naphtha, toluene, xylene, and kerosene products such as Isopar K and E.In general these lubricants will have a viscosity of at least 0.3centipoise at 25° C. and will be liquid under extrusion condition.Preferably they will contain paraffins, naphthenes and aromatics andsmall amounts of olefin.

In addition, the resins of this invention exhibit an unusualinsensitivity to stretch rate. Most fine powder resins exhibit varyingstretch performance properties as stretch rates are varied. Butsurprisingly when the stretch rate of a resin of this invention wasvaried between 10% per second and 100% per second, the stretched productexhibited no significant change in stretch uniformity or surfacesmoothness at a lubricant loading level of 17 wt %. Specifically, theuniformity of stretch was at least 75%. This means that an ink mark madeat the center of a paste extruded beading before stretching did not movemore than 25% from the center of the stretched product.

In addition, the stress relaxation times of the resins of this inventionare significantly greater than for most other fine powder resins.

The resins of this invention are useful in any of the paste extrusionapplications that known tetrafluoroethylene fine powder resins areuseful.

TEST PROCEDURES

(1) Raw Dispersion (Primary) Particle Size (Avg)

RDPS was determined from the absorbance (scattering) of a dilute aqueoussample at 546 millimicrons using a Beckman DU spectrophotometer and isbased on the principle that the turbidity of the dispersion increaseswith increasing particle size, as shown in U.S. Pat. No. 4,036,802.

(2) Standard Specific Gravity (SSG)

SSG was measured by water displacement of a standard molded testspecimen in accordance with ASTM D1457-69. The standard molded part wasformed by preforming 12.0 g of the powder in a 2.86 cm diameter die at apressure of 352 kg/cm², followed by the sintering cycle of the preformof heating from 300° C. to 380° C. at 2° C./min, holding at 380° C. for30 min, cooling to 295° C. at 1° C./min and holding at this temperaturefor 25 minutes, after which the specimen is cooled to 23° C. and testedfor specific gravity.

(3) Rheometer Pressure

Rheometer pressure was measured in accordance with ASTM D1457-81A,section 12.8, except that the resin was not sieved before mixing withthe kerosene lubricant and the preform was made in a 26 mm diameterextension tube at 300 psi.

(4) Specific Surface Area (SSA)

SSA was measured by a "Quantasorb" surface area analyzer sold by QuantaChrome Corp. The analyzer was calibrated by the B.E.T. method.

(5) Stretch Test

a. Preparation of Test Specimen

A sample of the resin was screened through a 2000 microns sieve. Onehundred grams of this resin was admixed with the desired amount ofIsopar K lubricant at room temperature by shaking in a glass jar of 6 cminside diameter and rolling for 4 min. at 64 rpm. It was then preformedat room temperature in a tube 26 mm diameter×23 cm long at 400 psi. Thepreform was then paste extruded at room temperature through an orifice2.4 mm in diameter into a uniform beading. Land length of the orificewas 5 mm. The extrusion speed was 84 cm/min. The angle of die was 30°.The beading was dried at 190° C. for 20 minutes.

b. Stretch Test

A beading of resin was cut and clamped at each end leaving a space of 50mm between clamps, and heated to 300° C. in a circulating air oven. Theclamps were then moved apart at the desired rate to the desired length.The stretched specimen was examined for uniformity of stretch, evenappearance and surface roughness. The % uniformity was calculated asfollows: ##EQU1## (6) Stress Relaxation Time

The specimen for the relaxation time measurement was made by stretchinga beading, as in Stretch Test, at 60% per second and 1500% totalstretch. Stress relaxation time is the time it takes for this specimento break when heated at 393° C. in the extended condition. For a shortperiod of time when the specimen is placed into the oven, thetemperature drops somewhat, e.g., to 375° C. and it takes about oneminute for the oven to return to 393° C. Stress relaxation time is thetime starting from placement of the test specimen into the oven.

EXAMPLES Example 1

A 36-liter polykettle was charged with 20.9 kg of demineralized water,600 g paraffin wax, 13 g ammonium perfluorooctanoate (C-8) dispersingagent, and 2.5 g succinic acid to reduce adhesion formation. Thecontents of the polykettle were heated to 75° C., evacuated of air, andN₂ purged. The contents of the polykettle were agitated at 46 RPM. Thetemperature was increased to 80° C. Tetrafluoroethylene (TFE) was thenadded to the polykettle after evacuation until the pressure was 2.75×10⁶Pa. Two hundred seventy (270) ml fresh, clear KMnO₄ solution (0.50 g/l)was added at 100 ml/min. After the polymerization began, as evidenced bya drop in pressure, tetrafluoroethylene was added to maintain thepressure at 2.75×10⁶ Pa. After 0.9 kg tetrafluoroethylene had reacted, asolution of 45 g C-8 in 1000 ml water was pumped in at 50 ml/min. After30 minutes from the start (kick-off) of the reaction, the temperaturewas raised to 90° C. Additional quantities of the KMnO₄ solution (0.50g/l) were added according to the following schedule:

    ______________________________________                                        KMnO.sub.4, ml                                                                            Time from kick off, min                                           ______________________________________                                        65          10                                                                65          20                                                                65          30                                                                ______________________________________                                    

The total KMnO₄ added was 0.2325 g. No KMnO₄ was added after 52% of theTFE had been polymerized. The reaction was 26% longer than if KMnO₄addition had continued until the end. After 14.1 kg tetrafluoroethylenehad reacted, the feed was stopped and the polykettle was vented,evacuated, and purged with N₂. The contents were cooled and dischargedfrom the polykettle. The supernatant wax was removed. The dispersion wasdiluted to 15% solids and coagulated in the presence of ammoniumcarbonate under high agitation conditions. The coagulated fine powderwas separated and dried at 150°-160° C. for three days.

The polymer properties are given in Tables I and II. The total reactiontime from tetrafluoroethylene pressure up to feed off was 74 mincompared to 123 min for Comparative Run B.

Example 2

Example 1 was repeated, except that:

the succinic acid amount was 0.5 g

additionally 0.1 g dibasic ammonium phosphate was added inhibitformation of MnO₂

the polymerization was carried out at a constant temperature of 90° C.

an additional 65 ml of KMnO₄ solution was also added after 40 min fromstart of reaction or kick-off (KO). The total KMnO₄ added was 0.265 g.No KMnO ₄ was added after 61% of the TFE had been polymerized. Thereaction was 34% longer than if KMnO₄ addition had continued to the end.

The polymer properties are given in Tables I and II. The extrusionpressure was high. The total reaction time was 89 min compared to 123minutes for Comparative Run B. The use of the phosphate had nodetrimental effect on the resin properties.

Example 3

Example 1 was repeated, except that:

the water amount was 20.0 Kg.

0.07 g ZnCl₂ was added.

no succinic acid was used.

120 ml of the 1.0 g/l KMnO₄ solution was initially added at 100 ml/min.

after 3.6 Kg TFE had reacted, another 120 ml of the 1.0 g/l KMnO₄solution was injected at 100 ml/min.

after 5 Kg TFE had reacted, the temperature was raised to 90° C.

after 8.7 Kg TFE had reacted, another 60 ml of the KMnO₄ solution wasinjected at 100 ml/min. No additional KMnO₄ was added after 62% of theTFE had been polymerized. The total KMnO₄ added was 0.30 g. The reactionwas 17% longer than if KMnO₄ addition had continued to the end.

The polymer properties are given in Tables I and II. The extrusionpressure was high. The total reaction time was 118 min.

This Example shows excellent stretch even under the condition of 23 wt.% of lubricant loading.

Comparative Run A

Example 1 was repeated, except that additional 65 ml of the KMnO₄solution was also added after 40 min and again after 50 min fromkick-off. The total KMnO₄ added was 0.2975 g. No KMnO₄ was added after94% of the TFE had been polymerized. The reaction did not slow down nearthe end of the polymerization.

The polymer properties are given in Tables I and II. The extrusionpressure was low. The total reaction time was 56 min. Even though thereaction time was shorter vs Example 1, the product performance wasunsatisfactory because of continuous supplying of the initiator to thereaction mixture until about the end of the polymerization.

Comparative Run B

The polykettle described in Example 1 was charged with 20 kgdemineralized water, 600 g paraffin wax, 13 g C-8 dispersant, and 10 gsuccinic acid. After a tetrafluoroethylene pressure of 2.75×10⁶ Pa wasobtained, 120 ml ammonium persulfate solution (1.0 g/l) was added at 100ml/min, at 75° C. After 0.9 kg tetrafluoroethylene had reacted, asolution of 45 g additional C-8 in 1000 ml water at 50 ml/min. wasadded. The temperature was maintained at 75° C. After 14.1 kgtetrafluoroethylene had reacted, the feed was stopped and the polykettlewas allowed to react down to 1.72×10⁶ Pa before venting. Fine powder wasobtained after processing as in Example 1.

The polymer properties are given in Tables I and II. The extrusionpressure was high but the total reaction time was 123 min. The stretchedspecimen severed during the test under the conditions of 23 wt. %lubricant loading level.

This example shows that the use of a commonly used initiator, such asammonium persulfate, causes substantially longer reaction time thanKMnO₄ and that the resin performance deteriorates at the higherlubricant loading level covered.

Comparative Run C

Comparative Run B was repeated, except that:

19 kg water was precharged

60 ml ammonium persulfate solution (1.0 g/l) was added aftertetrafluoroethylene pressure up

a polymerization temperature of 90° C. was used

a total 16.36 kg tetrafluoroethylene was reacted

the polykettle was vented immediately

The polymer properties are given in Tables I and II. The extrusionpressure was unsatisfactory. The total reaction time was 93 min.

This example shows that with a commonly used initiator, such as ammoniumpersulfate, a short reaction time caused by using higher temperature isaccompanied by inferior resin performance.

Example 4

A 36-liter polykettle was charged with 20.9 kG of demineralized water,600 g paraffin wax, 7 g ammonium perfluorooctanoate (C-8) dispersant, 5g succinic acid, 0.1 g diammonium hydrogen phosphate, and 0.40 g zincchloride. The contents of the polykettle were heated to 70° C., andrepeatedly evacuated and purged with TFE. After the final evacuation, 6ml of perfluorobutylethylene (PFBE) were added to the kettle. Thecontents of the kettle were agitated at 46 RPM. The temperature wasincreased to 80° C. TFE was then added to the kettle until the pressurewas 2.75×10⁶ Pa. Two hundred seventy (270) ml fresh, clear KMnO₄solution (0.50 g/1) were added at 100 ml per minute. After thepolymerization began, as evidenced by a drop in pressure, TFE was addedto maintain the pressure at 2.75×10⁶ Pa. After 0.9 kg TFE had reacted, asolution of 51 g C-8 in 1000 ml water was pumped in at 100 ml per min.After 5 min from the start of the reaction, the KMnO₄ solution (0.5 g/l)was pumped in at the rate of 6.5 ml/min until 7.7 kg TFE had reacted.The total KMnO₄ added was 0.26 g. After 14.1 kg TFE had reacted, thefeed was stopped and the kettle was vented, evacuated and purged withN₂. The contents were cooled and discharged from the kettle. Thesupernatant wax was removed. The dispersion was diluted to 15% solidsand coagulated under high agitation conditions. The coagulated finepowder was separated and dried at 150°-160° C. for 3 days. 0.02% PFBEwas present.

No KMnO₄ was added after 55% of the TFE had been reacted. The reactionwas extended 49% over that if KMnO₄ addition had continued to the end.

The polymer properties are given in Tables I and II.

The PFBE content in the resin was determined by Fourier Transform (FT)IR spectroscopy. Ten mil cold pressed films were prepared and spectrawere obtained on Nicolet 7000 FT IR spectrophotometer at a resolution of4 cm⁻¹. The --CH₂ -- bending vibration at 880 cm⁻¹, was used, calibratedusing NMR analysis. The absorbance at 880 cm⁻¹ was calculated by takingthe difference between absorbances at 888 and 880 cm⁻¹. For PFBEcomonomer, the calculation used is as follows: ##EQU2## wheret=thickness in mils and A=absorbance.

                                      TABLE I                                     __________________________________________________________________________                               Comparative Runs                                              Ex. 1                                                                             Ex. 2                                                                             Ex. 3                                                                             Ex. 4                                                                             A   B   C                                          __________________________________________________________________________    Total Reaction                                                                           74  89  118 99  56  132 93 (2)                                     Time, (Min) (1)                                                               RDPS, micron                                                                             0.210                                                                             0.234                                                                             0.258                                                                             0.216                                                                             0.198                                                                             0.252                                                                             0.237                                      SSG        2.161                                                                             2.159                                                                             2.160                                                                             2.146                                                                             2.169                                                                             2.166                                                                             2.168                                      Specific Surface                                                                         10.4                                                                              8.0 8.6 10.7                                                                              7.9 11.1                                                                              10.5                                       Area, m.sup.2 /g                                                              Rheometer Pressure,                                                                      349 379 368 449 239 390 295                                        kg/cm.sup.2 (RR = 400:1)                                                      Stress Relaxation                                                                        615 690 690 510 (3) (3) 555                                        Time (seconds)                                                                __________________________________________________________________________     (1) From TFE pressure up to TFE feed off. TFE reacted is 14.1 kg              (2) TFE reacted is 16.36 kg                                                   (3) Sample broke during stretching                                       

                  TABLE II                                                        ______________________________________                                        In the Table, the numbers show the % uniformity                               of stretch at the conditions described and the letter                         indicates surface smoothness at the conditions shown.                                               Comparative                                                                   Runs                                                              Ex. 1                                                                              Ex. 2  Ex. 3  Ex. 4                                                                              A    B    C                                 ______________________________________                                        Uniformity of                                                                             75A    80A    83A  88A  D    78B  61C                             stretch at lubri-                                                             cant loading of 17%                                                           and stretch rate of                                                           22% per second (4)                                                            Uniformity of                                                                             84B    80B    89A  80B  45C  D    72C                             stretch at lubri-                                                             cant loading of 23%                                                           and stretch rate of                                                           100% per second (5)                                                           Uniformity of                                                                             95A    92A    85A  90A  24C  97A  79B                             stretch at lubri-                                                             cant loading of 17%                                                           and stretch rate of                                                           100% per second (5)                                                           Uniformity of                                                                             75B    83B    94B  77B  D    D    72C                             stretch at lubri-                                                             cant loading of 23%                                                           and stretch rate of                                                           22% per second (4)                                                            Uniformity of                                                                             77B    84B    94A  83B  D    D    D                               stretch at lubri-                                                             cant loading of 17%                                                           and stretch rate of                                                           10% per second (5)                                                            Uniformity of                                                                             75B    96A    97B  97B  65C  68C  46C                             stretch at lubri-                                                             cant loading of 10%                                                           and stretch rate of                                                           100% per second (5)                                                           Uniformity of                                                                             79B    83B    95B  78B  D    D    54C                             stretch at lubri-                                                             cant loading of 25%                                                           and stretch rate of                                                           100% per second (5)                                                           ______________________________________                                         (4) 1000% total stretch                                                       (5) 1500% total stretch                                                       A Smooth even appearance                                                      B Slightly uneven appearance                                                  C Uneven appearance                                                           D Specimen severed (broke) during stretch test                           

I claim:
 1. An aqueous dispersion which contains dispersed particles ofpolymer, which polymer is a non-melt-fabricable tetrafluoroethylenepolymer prepared by the aqueous dispersion polymerization procedure inwhich the last portion of the polymerization is slowed down by stoppingaddition of initiator so that the end point is at least 5% longer thanif initiator addition is continued to the end of the reaction,characterized in that(a) the primary particle size is between 0.1 and0.5 microns; (b) the standard specific gravity is less than 2.190; (c)the rheometric pressure is at least 250 kg/cm² ; (d) the uniformity ofstrength is at least 75% throughout a lubricant loading range of 4weight percent which 4 weight percent range is within a lubricantloading level range between 10 and 25 weight percent, at a stretch rateof 100%/second, (e) the uniformity of stretch is at least 75% throughouta stretch rate of between 10 and 100%/second at a lubricant loadinglevel of 17%, and (f) the stress relaxation time is at least 400 second.2. The aqueous dispersion of claim 1 in which the polymer istetrafluoroethylene homopolymer.